The present invention relates to covalently and ionically crosslinked polymers, particularly to covalently and ionically crosslinked polymers comprising repeating units of the general formula
xe2x80x94Qxe2x80x94Rxe2x80x83xe2x80x83(1)
in which Q is a bond, oxygen, sulfur, 
the radical R is a divalent radical of an aromatic or heteroaromatic compound. The present invention further describes a process for preparing the covalently and ionically crosslinked polymers and also their use, particularly in fuel cells.
Polymers with repeating units of the general formula (I) are already known. They include, for example, polyarylenes, such as polyphenylene and polypyrene, aromatic polyvinyl compounds, such as polystyrene and polyvinylpyridine, polyphenylenevinylene, aromatic polyethers, such as polyphenylene oxide, aromatic polythioethers, such as polyphenylene sulfide, polysulfones, such as (copyright)Radel R, and polyether ketones, such as PEK. Moreover, they also embrace polypyrroles, polythiophenes, polyazoles, such as polybenzimidazole, polyanilines, polyazulenes, polycarbazoles, and polyindophenines.
Recently, the use of such polymers for producing membranes for use in fuel cells has become increasingly important. Polymers with basic groups, such as sulfonic acid groups and amino groups, in particular, are increasingly being described in the literature. The membranes are doped with concentrated phosphoric acid or sulfuric acid and serve as proton conductors in what are known as polyelectrolyte membrane fuel cells (PEM fuel cells). Such membranes allow the membrane electrode assembly (MEA) to be operated at relatively high temperatures and so increase the tolerance of the catalyst significantly toward the carbon monoxide which is produced as a by-product in the reformation, thereby substantially simplifying the reprocessing or purification of gas. Disadvantages of these membranes are their mechanical instability, with a low modulus of elasticity, a low tensile strength, and a low upper flow limit, and their relatively high permeability to hydrogen, oxygen, and methanol.
First attempts to solve these problems are disclosed in the documents DE 196 22 337, WO 99/02755, and WO 99/02756. DE 196 22 337 describes a process for producing covalently crosslinked ionomer membranes which is based on an alkylation reaction of sulfinate-functional polymers, polymer blends, and polymer (blend) membranes. The covalent network is resistant to hydrolysis even at relatively high temperatures. A disadvantage, however, is that, owing to the hydrophobic covalent network, the covalently crosslinked ionomers and ionomer membranes dry out easily and may therefore undergo severe embrittlement; as a result, they are of only limited suitability for applications in fuel cells, especially at relatively high temperatures.
The documents WO 99/02756 and WO 99/02755 disclose ionically crosslinked acid-base polymer blends and polymer (blend) membranes. One advantage of the ionically crosslinked acid-base blend membranes is that the ionic bonds are flexible, even at relatively high temperatures the polymers/membranes do not dry out so easily, owing to the hydrophilicity of the acid-base groups, and therefore the polymers/membranes do not undergo embrittlement even at relatively high temperatures. The ionically crosslinked ionomer (membrane) systems described in these documents, however, have the disadvantage that the ionic bonds part in the temperature range between 60 and 90xc2x0 C. and from this temperature range on the polymers/membranes begin exorbitantly to swell. Consequently, these membranes too are poorly suited to applications in fuel cells, especially at relatively high temperatures upward of 80xc2x0 C.
In the light of the prior art it is now an object of the present invention to provide a crosslinked polymer having improved properties. The polymer of the invention is to have a low specific volume resistance, preferably less than or equal to 100 xcexa9cm at 25xc2x0 C., and to exhibit low permeability for hydrogen, oxygen, and methanol.
Furthermore, it is to have a very good mechanical stability, in particular an improved modulus of elasticity, a higher tensile strength, and improved swelling properties. It should preferably swell by less than 100% in deionized water at a temperature of 90xc2x0 C.
A further object was to specify a crosslinked polymer which can be used in fuel cells. The crosslinked polymer ought in particular to be suitable for use in fuel cells upward of 80xc2x0 C., in particular upward of 100xc2x0 C.
A further object of the invention was to provide a process for preparing the crosslinked polymer that can be carried out simply, inexpensively, and on an industrial scale.
These objects and further objects, which are not mentioned explicitly but can readily be derived or inferred from the circumstances discussed introductorily herein, are achieved by means of a covalently and ionically crosslinked polymer having all of the features of claim 1. Appropriate modifications of the crosslinked polymer of the invention are protected in the subclaims which refer back to claim 1. Processes for preparing the crosslinked polymer of the invention are described in the process claims, while the claims of the use category protect preferred uses of a crosslinked polymer of the invention.
By virtue of the fact that a covalently and ionically crosslinked polymer comprising repeating units of the general formula (I) is made available which is distinguished in that
a) the radical R has at least in part substituents of the general formula (4A), (4B), (4C), (4D), (4E), (4F), (4G) and/or (4H) 
where the radicals R1 independently of one another are a bond or a group having 1 to 40 carbon atoms, preferably a branched or unbranched alkyl or cycloalkyl group or an optionally alkylated aryl group,
M is hydrogen, a metal cation, preferably Li+, Na+, K+, Rb+, Cs+, or an optionally alkylated ammonium ion, and
X is a halogen or an optionally alkylated amino group,
b) the radical R has at least in part substituents of the general formula (5A) and/or (5B) 
in which R2, R3, R4 and R5 independently of one another are a group having from 1 to 40 carbon atoms, preferably a branched or unbranched alkyl or cycloalkyl group or an optionally alkylated aryl group, it being possible for at least two of the radicals R2, R3, and R4 to be closed to form an optionally aromatic ring,
and/or the radical R is at least in part a group of the general formula (5C) and/or (5D) 
and
c) the radical R has at least in part bridges of the general formula (6) 
which join at least two radicals R to one another,
Y being a group having from 1 to 40 carbon atoms, preferably a branched or unbranched alkyl or cycloalkyl group or an optionally alkylated aryl group,
Z is hydroxyl, a group of the general formula 
or a group having a molecular weight of more than 20 g/mol, composed of the optional components H, C, O, N, S, P, and halogen atoms, and
m is an integer greater than or equal to 2,
it is possible in a manner which was not immediately foreseeable to make available a crosslinked polymer having improved mechanical properties, in particular a higher modulus of elasticity, an improved tensile strength, and improved swelling properties.
At the same time the crosslinked polymer of the invention display a number of further advantages. These include, among others:
The doped polymer membranes have a low specific volume resistance, preferably less than or equal to 100 xcexa9m at 25xc2x0 C.
The doped polymer membranes possess only a low permeability for hydrogen, oxygen and methanol.
Even an extremely thin membranes of the crosslinked polymer of the invention, with a total thickness of between 10 and 100 xcexcm, possess sufficiently good material properties at 80xc2x0 C., in particular a very high mechanical stability and a low permeability for hydrogen, oxygen and methanol.
The doped polymer membrane is suitable for use in fuel cells upward of 80xc2x0 C., in particular under standard pressure.
The doped polymer membrane can be produced simply, on an industrial scale, and inexpensively.
In accordance with the present invention the polymer is ionically and covalently crosslinked. In accordance with the invention, crosslinked polymers are those polymers whose linear or branched macromolecules, which are of the same or different chemical identity and are present in the form of collectives, are linked to one another to form three-dimensional polymer networks. In this case the crosslinking may be effected both by way of the formation of covalent bonds and by way of the formation of ionic bonds. Further details may be taken from the technical literature, examples being C D Rxc3x6mpp Chemie Lexikonxe2x80x94Version 1.0, Stuttgart/New York: Georg Thieme Verlag 1995, entry on xe2x80x9cCrosslinkingxe2x80x9d and the literature cited in that section.
The crosslinked polymer of the invention has repeating units of the general formula (1), especially repeating units corresponding to the general formulae (1A), (1B), (1C), (1D), (1E), (1F), (1G), (1H), (1), (1J), (1K), (1L), (1M), (1N), (1O), (1P), (1Q), (1R), (1S) and/or (1T): 
Independently of one another here the radicals R6 which are identical or different, are 1,2-phenylene, 1,3-phenylene, 1,4-phenylene, 4,4xe2x80x2-biphenyl, a divalent radical of a heteroaromatic, a divalent radical of a C10 aromatic, a divalent radical of a C14 aromatic and/or a divalent pyrene radical. An example of a C10 aromatic is naphthalene; of a C14 aromatic, phenanthrene. The substitution pattern of the aromatic and/or heteroaromatic is arbitrary, in the case of phenylene, for example, R6 may be ortho-, meta- and para-phenylene.
The radicals R7, R8, and R9 designate monovalent, tetravalent and trivalent aromatic or heteroaromatic groups, respectively, and the radicals U, which are identical within a repeating unit, are an oxygen atom, a sulfur atom or an amino group which carries a hydrogen atom, a group having 1-20 carbon atoms, preferably a branched or unbranched alkyl or alkoxy group, or an aryl group as a further radical.
The polymers with repeating units of the general formula (1) that are particularly preferred in the context of the present invention include homopolymers and copolymers, examples being random copolymers, such as (copyright)Victrex 720 P and (copyright)Astrel. Especially preferred polymers are polyaryl ethers, polyaryl thioethers, polysulfones, polyether ketones, polypyrroles, polythiophenes, polyazoles, phenylenes, polyphenylenevinylenes, polyanilines, polyazulenes, polycarbazoles, polypyrenes, polyindophenines and polyvinylpyridines, especially polyaryl ethers: 
Especially preferred in accordance with the invention are crosslinked polymers with repeating units of the general formula (1A-1), (1B-1), (1C-1), (1I-1), (1G-1), (1E-1), (1H-1), (1I-1), (1F-1), (1J-1), (1K-1), (1L-1), (1M-1) and/or (1N-1).
In the context of the present invention, n designates the number of repeating units along one macromolecule chain of the crosslinked polymer. This number of the repeating units of the general formula (1) along one macromolecule chain of the crosslinked polymer is preferably an integer greater than or equal to 10, in particular greater than or equal to 100. The number of repeating units of the general formula (1A), (1B), (1C), (1D), (1E), (1F), (1G), (1H), (1I), (1J), (1K), (1L), (1M), (1N), (1O), (1P), (1Q), (1R), (1S) and/or (1T) along one macromolecule chain of the crosslinked polymer is preferably an integer greater than or equal to 10, in particular greater than or equal to 100.